Patent GB-1297296-A (GB)
IPC: G01N25/68
Filed: 1970-11-27
Abstract: 1297296 Dewpoint hygrometer EG & G Inc 27 Nov 1970 [18 Dec 1969] 56425/70 Heading, G1A In a dewpoint hygrometer employing two photo-sensitive resistors within a bridge circuit and and a mirror whose temperature is maintained at dewpoint by a servo-loop operating a cooling device attached to the mirror, the effects of the build up of contaminants on the mirror is neutralized by including a variable resistor in the bridge, the resistor being continuously or periodically adjusted to rebalance the bridge. In one embodiment, Fig. 3 light from a source 32 is directed onto a mirror 12 and the reflected and scattered light is received by photo-sensitive resistors 64 and 66 respectively. The thickness of the condensate on the mirror will determine the relative proportions of scattered and reflected light. The photo-sensitive resistors 64 and 66 are connected as shown, in a bridge 120 the output from which is fed via a pre-amplifier 126 and a power control amplifier 128 to a current regulator 46 which controls the cooling current from a current supply 44 for a thermoelectric cooler 14. The bridge is initially balanced for a particular thickness of condensate by means of variable resistors 124 and 82, and the arrangement is such that the mirror will then be maintained at the dewpoint temperature of the gas flowing past its reflective surface. The temperature of the mirror is monitored by means of a thermocouple 48, an amplifier 50 and a meter 52. The build up of contaminants on the mirror's reflective surface is neutralized by employing an alternating current generator 130 so that an oscillatory current is added to the cooling current. This results in small fluctuations in the mirror's mean temperature and a corresponding fluctuation in the thickness of the condensate which in turn influences the resistances of photo-sensitive resistors 64 and 66. The amplitude of the resulting alternating current is measured by a detector 136 with the aid of a turned amplifier 134. The detector's output drives a meter 138 and a lamp 140 which irradiates a further photo-resistor 122 connected in the bridge as shown. Thus any unbalancing of the bridge by contaminants causes a change in the temperature of the mirror, and, thus a change in the thickness of the condensate is sensed as a change in amplitude by detector 136 whose output will change accordingly and thereby change the resistance of photo-resistor 122. In a similar embodiment Fig. 4 (not shown), the A.C. current generator 30 is omitted, and instead the gain feedback system, provided by preamplifier 128, is increased to the point where slight instability occurs and the system becomes self oscillatory. / In a further embodiment, Fig. 2, which periodically adjusts the bridge circuit to neutralize the effect of contaminants, a motor B2 is employed to control a timing switch 51 energizing a first relay K1 which controls contacts K1B interrupting the cooling current servo-loop and allows the mirror to increase in temperature and evaporate the condensate. A second relay K2 is connected to an electronic switch 92 which is responsive to a predetermined error signal from the bridge 60 greater than a predetermined value. The second relay controls the circuit of a balance B1 motor which drives a potentiometer 68 in the bridge circuit 60. Thus the potentiometer is adjusted till the bridge is balanced and the error signal is reduced sufficiently to de-activate relay K2.